Any reliable and comprehensive GPU/CPU cycles rendering study
I don't think the GPU power is bottlenecked by CPU in any way. You either render using CPU or GPU. If you're using a CPU you need to set the tile size to some small number like 16 x 16 or 20 x 20 so that each CPU thread will be responsible for one of these tiles and the process becomes more efficient this way. When using a GPU for rendering you need to set the tile size to a higher number such as 224 x 224 and so on so that you can benefit from using a GPU.

Blender OpenData might give you some ideas on what GPUs/CPUs are better for rendering.